FIGS. 1A and 1B depict conventional magnetic elements 10 and 10′ that can be used to read magnetic recording data. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional conductive spacer layer 16 and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional conductive spacer layer 16 is nonmagnetic. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization 19 of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional insulating barrier layer 16′ and a conventional free layer 18′ having a changeable magnetization 19′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
Depending upon the orientations of the magnetization 19/19′ of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetization 19/19′ of the conventional free layer 18/18′ is parallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is low. When the magnetization 19/19′ of the conventional free layer 18/18′ is antiparallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is high.
In order to read data, the conventional magnetic element 10/10′ is brought into proximity with a magnetic media near the air-bearing surface (ABS). The conventional free layer 18/18′ responds to the varying magnetic field from the recording media. As a result, the magnetization 19/19′ of the conventional free layer 18/18′ will be substantially parallel or antiparallel to the magnetization of the pinned layer 14/14′. To read the data stored in the recording media, therefore, the resistance of the conventional magnetic element 10/10′ is determined. The resistance of the conventional magnetic element 10/10′ is sensed by driving a current through the conventional magnetic element 10/10′. Current may be driven in a CIP (current-in-plane) configuration or in a CPP (current perpendicular to the plane) configuration. In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 10/10′ (up or down, as seen in FIG. 1A or 1B).
In higher density recording applications beyond one hundred gigabits per square inch, the CPP configuration is an attractive candidate for use in a read head. However, when the sizes of the conventional magnetic elements 10/10′ are shrunk down to deep submicron sizes (approximately <0.2 μm), a newly discovered phenomenon called the spin transfer effect can become dominant. The spin transfer effect is described more fully in J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1-L5(1996). When a spin polarized current traverses a magnetic multilayer stack such as the conventional magnetic element 10/10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18/18′ may transfer a portion of their spin angular momentum to the conventional free layer 18/18′. As a result, a spin-polarized current can affect the direction of the magnetization 19/19′ of the conventional free layer 18/18′. If the current density is sufficiently high (approximately 107-108 A/cm2) and the conventional free layer 18/18′ is sufficiently thin, for instance, less than approximately ten nanometers for Co, the magnetization 19/19′ of the free layer may actually be switched due to spin transfer. According to Slonczewski's model, the switching current density Jc for the free layer of a simple trilayer spin transfer stack (for example, thick Co/Cu/thin Co) is proportional to:αtMs[Heff−2πMs]/g(θ)
where:
α=the phenomenological Gilbert damping constant;
t=the thickness of the free layer;
Ms=saturation magnetization of the free layer;
Heff=effective field for the free layer;
g(θ) reflects the spin-transfer efficiency
Thus, depending upon the values of the constants and fields described above, the magnitude of the spin transfer effect varies. The spin transfer effect can result in an asymmetric bias of the conventional free layer 18/18′ and in magnetic noise in a magnetic recording read head.
Accordingly, what is needed is a system and method for reducing noise due to the spin transfer effect. The present invention addresses such a need.